Optical scanners of the type shown in U.S. Pat. No. 4,512,659 to L. Galbraith are used to measure contamination of bare semiconductor wafers before a chip fabrication process begins. Contamination is frequently in the form of particulate matter, with particles ranging in size from very large millimeter size flakes to sub-micron specs of dust or dirt.
In surface scanning, the general state of contamination is an important consideration, but more particularly, the actual particle count is desired information. Most surface scanners scan back and forth in a single direction, while a wafer being inspected is moved beneath the scanning beam in a perpendicular direction. This establishes an x-y scan over a wafer surface. The scan controller usually establishes a wafer-shaped scan pattern so that the scanning beam is always on the wafer surface and does not encounter a wafer edge.
One of the problems encountered in optical surface scanning is that light scattered from a feature on a surface varies depending on the position of the feature relative to the scan center of the beam. There are at least two reasons for such variation. A first reason is that light intensity from the scan center falls off as the square of the distance from that center. Thus a light scattering feature closer to the scan center will present a stronger scattering signal to a remote detector than the same feature further away. A second reason is that the scanning beam spot which falls on the surface varies in cross sectional shape across the surface. A beam which is circular in shape at the center of the surface becomes elliptical at the edge of the surface. Such changes in beam spot shape cause changes in the amount of light scattered from a feature. Additionally, if the scan center is not directly over the center of the surface, there will be an appreciable difference in scattering from opposed edges of the surface. These problems are obviated in telecentric scanners, but many of the surface scanners presently in use are not telecentric.
There is a need for a reference substrate for checking particle count in a surface scanner where beam illumination varies over the surface and where surface edges may be misaligned in relation to the scan center.